Cisco ASR 5000 Series, Обзор продукта

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GGSN Support in GPRS/UMTS Wireless Data Services
▀ Features and Functionality - Optional Enhanced Feature Software
▄ Cisco ASR 5000 Series Product Overview
OL-22938-02
Traffic Policing and Rate Limiting
Allows the operator to proportion the network and support Service-level Agreements (SLAs) for customers.
The Traffic-Policing/Shaping feature enables configuring and enforcing bandwidth limitations on individual PDP
contexts of a particular 3GPP traffic class. Values for traffic classes are defined in 3GPP TS 23.107 and are negotiated
with the SGSN during PDP context activation using the values configured for the APN on the GGSN. Configuration and
enforcement is done independently on the downlink and the uplink directions for each of the 3GPP traffic classes.
Configuration is on a per-APN basis, but may be overridden for individual subscribers or subscriber tiers during
RADIUS authentication/authorization.
A Token Bucket Algorithm (a modified trTCM, as specified in RFC2698) is used to implement the Traffic-Policing
feature. The algorithm measures the following criteria when determining how to mark a packet.
Committed Data Rate (CDR) : The guaranteed rate (in bits per second) at which packets may be
transmitted/received for the subscriber during the sampling interval.
Peak Data Rate (PDR) : The maximum rate (in bits per second) that packets may be transmitted/received for the
subscriber during the sampling interval.
Burst-size : The maximum number of bytes that may be transmitted/received for the subscriber during the
sampling interval for both committed (CBS) and peak (PBS) rate conditions. This represents the maximum
number of tokens that can be placed in the subscriber's ―bucket‖. Note that the committed burst size (CBS)
equals the peak burst size (PBS) for each subscriber.
Tokens are removed from the subscriber's bucket based on the size of the packets being transmitted/received. Every
time a packet arrives, the system determines how many tokens need to be added (returned) to a subscriber's CBS (and
PBS) bucket. This value is derived by computing the product of the time difference between incoming packets and the
CDR (or PDR). The computed value is then added to the tokens remaining in the subscriber's CBS (or PBS) bucket. The
total number of tokens can not be greater than the configured burst-size. If the total number of tokens is greater than the
burst-size, the number is set to equal the burst-size. After passing through the Token Bucket Algorithm, the packet is
internally classified with a color, as follows:
There are not enough tokens in the PBS bucket to allow a packet to pass, then the packet is considered to be in
violation and is marked ―red‖ and the violation counter is incremented by one.
There are enough tokens in the PBS bucket to allow a packet to pass, but not in the CBS ―bucket‖, then the
packet is considered to be in excess and is marked ―yellow‖, the PBS bucket is decremented by the packet size,
and the exceed counter is incremented by one.
There are more tokens present in the CBS bucket than the size of the packet, then the packet is considered as
conforming and is marked ―green‖ and the CBS and PBS buckets are decremented by the packet size.
The APN on the GGSN can be configured with actions to take for red and yellow packets. Any of the following actions
may be specified:
Drop : The offending packet is discarded.
Transmit : The offending packet is passed.
Lower the IP Precedence : The packet's ToS octet is set to ―0‖, thus downgrading it to Best Effort, prior to
passing the packet.